The present disclosure relates generally to the field of medical systems for detecting seizures. More particularly, the disclosure relates to systems, methods and apparatuses for using an accelerometer based system for detecting seizure and non-seizure states in patients experiencing seizures.
The embodiments described herein relate generally to the field of medical systems for detecting seizures. “A seizure is an abnormal, unregulated electrical charge that occurs within the brain's cortical gray matter and transiently interrupts normal brain function.” The Merck Manual of Diagnosis and Therapy, 1822 (M. Beers Editor in Chief, 18th ed. 2006) (“Merck Manual”). Epilepsy is a chronic disease characterized by such seizures, but not caused by an event such as a stroke, drug use, or physical injury. Seizures may vary in frequency and scope and may range from involving no impairment of consciousness at all to complete loss of consciousness. Typically, a seizure resolves within a few minutes and extraordinary medical intervention, other than that needed for the comfort of the patient and to promote unobstructed breathing, is not needed. (See, generally, Merck Manual at 1822-1827, incorporated herein by reference.)
If a patient is aware that a seizure is beginning, the patient may prepare for the seizure by ceasing activity that may be dangerous should a seizure begin, assuming a comfortable position, and/or alerting friends or family. In some patients, an implanted neurostimulator, such as that described in U.S. Pat. No. 5,304,206, incorporated herein by reference, may be activated, which may allow the patient to avoid the seizure, limit the seizure severity and/or duration, or shorten the patient's recovery time. Some patients may not experience onset symptoms indicating that a seizure is imminent or beginning In addition, others may not be aware a seizure has taken place. A record of the frequency, duration, and severity of seizures is an important tool in diagnosing the type of seizures that are occurring and in treating the patient.
Accelerometers have been known for detecting movement in seizure patients. See, for example, U.S. Pat. No. 5,304,206, column 8, lines 28-33. (“A motion sensor is provided within the bracelet for automatically detecting movements by the patient. The motion sensor portion of the detection system 78 (FIG. 6) may be of any known type, such as an accelerometer or a vibration sensor, but preferably, is a contact-type sensor as shown in principal part in FIG. 9.”) An accelerometer measures “proper acceleration” of an object. Proper acceleration is different than the more familiar concept of “coordinate acceleration.” Coordinate acceleration is a change in velocity of an object with respect to its surroundings, such as an automobile accelerating from zero to 60 miles per hour in a given number of seconds.
By contrast, proper acceleration is the physical acceleration of an object relative to an observer who is in free fall. Proper acceleration is measured in units of “g-force” or gravity/seconds2. Proper acceleration can also be considered to be weight of an object per unit of mass. When an object sits motionless on the ground, its coordinate acceleration is zero. But to determine the object's proper acceleration, one compares the object to the observer in free fall, who is falling towards the center of the Earth. A force acts on the motionless object that is not acting on the observer in free fall: the force of the Earth is pushing up on the object, holding it in place, so the motionless object has a proper acceleration of 1 gravity/second2.
An accelerometer may be used to determine a sudden change of position of a person, which might be indicative of a seizure in the person with physical symptoms. Both the amplitude of the change in position and the frequency of the change in position could be important indications of a seizure, depending in part on how seizures affect a particular patient. Of course, a change of position may have a non-seizure cause, as people sometimes engage in strenuous activities. Most algorithms used to analyze accelerometer data are highly complex, making them inconvenient to use. The use of some accelerometer algorithms requires training and position correction. Most of the current algorithms used to analyze accelerometer data also require that the accelerometer be held in a proper orientation.
Accordingly, a need is present for methods, systems and apparatuses to better detect seizures and/or overcome issues discussed above.